FIELD OF INVENTION

The present invention relates to an air drying amino acrylic top coat and preparation thereof. More particularly this invention pertains to an air drying coating composition comprising partially hydroxylated acrylate polymer and alkoxylated amino resin(s). The coating is free from isocyanates and other toxic ingredients and is useful as a topcoat for steel structures exposed to hostile environmental pollution due to its excellent chemical, weather and UV resistance. The said coating composition may be pigmented and is usually formulated in volatile organic aromatic solvent. It cures rapidly under ambient conditions to provide a glossy barrier coating when applied as topcoat, preferably on a corrosion resistant base coat for protection of steel/metalized steel structures exposed to industrial pollutants and/or coastal salinity.

BACKGROUND OF INVENTION

Thermosetting coatings based on acrylic polyols and isocyanate resins are well known for automotive top coat and refinish applications. These coatings are commonly described as polyurethane-acrylic finish coats, which offer UV and chemical resistance in clear or coloured finish coats, especially for automotive bodies. Similar products are often applied as top coat over high build epoxy based tie coats in a ternary coating scheme to guard against UV attack in exterior protection of steel structures in corrosive areas. However, these coatings suffer from major drawbacks due to the isocyanate resins used which have limited to high toxicity, and, pose difficulties in recoating pre-coated structures during maintenance. Attempts have been made to retain the acrylic structure for its universal acceptability in exterior applications and replace isocyanates through modification of the acrylic polymer. One approach has been incorporation of a polyester extended acrylate monomer in the acrylic polymer backbone which can be cross linked with amino resins with or without isocyanates as disclosed in United States Patent Application No.US2009/0162559 Al. This heat reactive composition is recommended as a base coat on phosphated and primed steel substrate, preferably as a wet-on- wet coating for automotive applications. The thickness recommended is less than 40 micrometres and this coating may be followed by a clear coat. Some of these formulations are water soluble/extendable. An isocyanate free composition has been disclosed in US Patent Application No.US4525521 A, since expired, which incorporates pendant amino ester/amino ester hydroxy groups and glycidyl acrylic groups in binder resin/polymer. The coating composition is air drying and provides resistance to water, gasoline products, etc. However, no performance tests with regard to industrial effluents have been mentioned. As such this and related compositions may be useful as finish/refinish coatings in automotive applications, but may not be adaptable for application in protecting industrial infrastructure. Hence there is a definite need for an exterior top coat that is:

1. Devoid of isocyanates or any other toxic ingredient;

2. Resistant to a variety of acidic and alkaline chemicals, salinity, gasoline products, mineral or other fuel oils, etc.;

3. Resistant to aggressive weather and aesthetically pleasing, dispensing with clear coat in ternary coating schemes; and very importantly

4. Recoatable for easy maintenance;

5. Cost-effective; and

6. User friendly requiring no special handling.

The principal object of the present invention is to provide an air drying amino acrylic top coat and the procedure to arrive at the desired product.

A further object of this invention is to eliminate usage of toxic ingredients to ensure safety and well-being of workmen and actual users.

The foregoing objectives are achieved by the present invention which comprises a two-component system which is characterized (by weight, based on 100% solids of all resinous constituents) in that - (i) Component 1 is constituted by (a) acrylic polyol - 50% max, (b) paratoluene sulphonic acid - 1.25% max,

(ii) Component 2 is constituted by (a) partially alkoxylated (with carbon atoms not exceeding 4 in the alkyl group) formaldehyde resin - 13% max, (b) methylated melamine formaldehyde resin - 2.25 % max, and

(iii) Organic solvents selected from normal and iso-butanol (15% max), and xylene (50% max) are present in both components.

(iv) Component 1 may include pigment(s), extender(s), and UV absorbers, depending on application and user preference.

It was found rather unexpectedly that an organic solvent soluble alkoxylated urea formaldehyde resin combines with an acrylic polyol to provide an ambient curing top coat. Moreover, ambient cure can be regulated by adjusting concentration of a suitable acid catalyst to obtain a working life of the catalysed liquid coating of at least 4 hours under ambient conditions (30°C) which is a distinct improvement over polyurethane coatings. Notably, the amino resin used which is normally recommended for wood coating applications and also for indoor use, in combination with the chosen acrylic polyol, is well suited to continuous exterior application especially in the pigmented form. The coating demonstrates excellent chemical resistance and barrier properties in dry film thickness of 50-100 micrometres and is serviceable up to 120°C.

This invention provides a two-component ambient curing acrylic coating composition that can serve as top coat to steel/metalized structures, protected from interfacial corrosion by a suitable base coat. Importantly it eliminates toxic isocyanates altogether as crosslinking agent and film hardening and curing take place through condensation of pendant hydroxyls of the acrylic polymer with alkoxy groups of the alkoxylated urea formaldehyde resin, catalysed by an acid catalyst such as p-toluene sulfonic acid, under ambient conditions. The said coating composition contains volatile organic solvent and the dry film obtained after ambient cure is hydrophobic and weather resistant. If necessary, protection from UV may be further enhanced, particularly in high UV exposure zones, by incorporating UV absorbers such as "TINUVIN" products of BASF in the coating composition. The coating has been extensively tested on ferrous and non-ferrous substrates which were pre-coated with a corrosion resistant, self priming base coat. It imparts excellent barrier properties in thickness ranging from 50 to 100 micrometres and chemical resistance against salinity, industrial chemicals comprising acids and alkalis, mineral and fuel oils and also their combustion products. The subject air drying coating formulation was made as a two-component system, to be mixed prior to application. Both components contain a volatile organic solvent such as xylene. The coating upon application air dries into a glossy aesthetically pleasing finish. The composition may be formulated as a clear coat or pigmented to match industrial colour shades. When used as a top coat on a surface tolerant base coat, the coating scheme will enhance life of new as well pre-erected structures in industrial and/or coastal environments without elaborate surface preparation and drastically reduce their life cycle costs. Importantly it may be re-applied on previously top coated surface at any time, thus facilitating maintenance of coated structures. The top coat withstands sustained exposure up to 120°C.

Acrylic polyols used in the subject composition may be synthesised in several ways. For example, these may be obtained by copolymerization of an allylic alcohol and an unsaturated acrylic monomer. Alternatively hydroxy alkyl acrylate/methacrylate and unsaturated vinyl or acrylate monomers may be copolymerized. Solution polymerization in a volatile organic solvent is most often used to control temperature of polymerization. The temperature, choice and concentration of co-monomers, chain transfer agent, solvent, and free radical initiator, all govern the kinetics, hydroxyl value, and, molecular properties such as average molecular weight and distribution of the co- polymer, and viscosity of the hydroxylated acrylate polymer solution. The solvent medium may be chosen from alkyl acetates and/or aromatics. A solvent that is common to both the cross linkable acrylic polyol and cross linking alkoxy amino resin is preferred for the subject coating composition as it promotes homogeneity and stability of the two- component mix for topcoat application. Xylene was used as the solvent medium in the composition as both acrylic polyol and urea resins employed xylene as sole, or, one of the solvents. The hydroxylated acrylic polyols suitable for this invention contain pendant ester and hydroxyl groups and no amino ester groups. These may have solids percentage ranging from 50-80 in pure xylene or xylene mixed with butyl acetate with a solution viscosity of 1500-5000cps and a hydroxyl number on solid resin of 80-100. The preferred polyol has 60% dissolved solids in xylene with a viscosity of 1500-3500 cps at 23°C and a hydroxyl number of 85 on solid resin.

The polyol solution may be used as part of a clear coat or coloured with a variety of organic/inorganic pigments, extenders such as micro fine silica, and special additives, if necessary, to form Component 1 of the subject composition which is catalysed by p-toluene sulfonic acid, added as a solution in very small quantity of n-butanol. Component 2 of the two-component system could be made from a mix of alkoxylated urea formaldehyde, and alkoxylated melamine formaldehyde, wherein the latter is added in small proportion to stabilize the composition. The urea formaldehyde condensation product contains imino, methylol and alkoxy groups. The alkoxy urea formaldehyde resin suited to this invention had 1 to 4 carbons in the alkyl group. The preferred choice of the urea resin was a partially butylated urea formaldehyde resin dissolved in a mix of iso-butanol and xylene. The solids percentage of this resin is 58-62% by weight and its viscosity ranges from 1200-2400 cps at 23°C. The melamine resin is preferably methylated, and its average degree of polymerization is 1.75. The solids percentage of the resin is above 98%.

The essential constituents of the amino acrylic air drying composition may be laid down as under, computed on the basis of 100% solid state by weight, and listing a range of individual weight of each of the constituents per 100 parts of the acrylic polyol:

1. Partially butylated urea formaldehyde - 23-26

2. Methylated melamine formaldehye - 1.5-4.5

3. P-toluene sulphonic acid - 1-2.5

Upon mixing the two components at ambient temperature (30°C approximately) a pot life of at least 4 hours was achieved. The liquid coating could be formulated either as a clear coating or as a pigmented coating with a solid percentage of 40-65% by weight and viscosity of 60-100 seconds measured by Ford Cup B4 at 30°C. The desirable dry film thickness (hereafter referred to as DFT) of the topcoat ranges from 50 to 100 microns, depending on the level and type of industrial pollutants in the environment.

GIST OF THE INVENTION

As narrated hereinbefore, the present invention may be summarized as an amino acrylic air-drying coating composition comprising: a) acrylic polyol resin b) an alkoxylated urea formaldehyde resin; c) an alkoxylated melamine formaldehyde resin; d) acid catalyst for accelerating ambient cure; e) organic solvents like xylene; f) chromate-free pigment(s), extenders such as micro fine silica; and g) optionally additives like "TINUVIN" UV absorbers

Preferred components used in the composition of this invention are: a) Acrylic polyol resin containing 60% dissolved solids in xylene with a viscosity of 1500-3500 cps at 23°C and a hydroxyl number of 85 on solid resin; b) Partially butylated urea formadehyde resin containing 60% solids in a mix of xylene and iso-butanol; c) Methylated melamine formaldehyde resin containing more than 97% solids; and d) p-Toluene sulphonic acid as acid catalyst

This invention also includes within its scope the procedure for arriving at the subject coating composition which can be briefly outlined as under:

Component 1 of the composition was made by adding the requisite pigments, extenders, and additives if any, to the acrylic polyol resin and grinding in a ball mill or attritor to achieve the requisite fineness of grind. P-toluene sulphonic acid was dissolved in n-butanol and added to the mill base along with requisite amount of xylene to obtain the final consistency.

Component 2 of the composition was made by mixing the alkoxylated amino resins in specified proportions and level of solids adjusted by adding xylene. Solid percentage of the final composition after mixing parts 1 and 2 ranged from 40-65 by weight for clear coat and pigmented compositions. The summarized narration of the constituent components used in forming the subject air drying coating composition may be suitably elaborated in the following examples which are given by way of illustration and not by way of limitation.

EXAMPLE 1

The following illustration of a clear coat comprising a two-component system comprises the ingredients in the specified range by weight.

COMPONENT 1 COMPONENT 2

The proportions of Component 1 and Component 2 required to be mixed for final clear coat formulation will follow the requirements on a 100% solid basis as outlined before.

EXAMPLE 2

A specific satin blue coloured two-component coating composition was made using following ingredients with percentage of weight indicated for the individual ingredients.

COMPONENT 1

COMPONENT2 The above pigmented composition contained 55% solids by weight in

Components 1 and 2 individually. Part 1 was ground in a high speed attritor to Hegman scale of 7. 100 parts of Part 1 was mixed with 17 parts of Part 2 to obtain the final air drying liquid coating composition. The viscosity of the mix was 80 seconds as measured by Ford Cup B4 at 30°C and its specific gravity was 1.20. This particular composition was used in all tests narrated hereafter in detail.

TESTS AND RESULTS

Two kinds of steel panels were used in these studies. Cold rolled steel (CRS) panels were cleaned with emery paper and washed with xylene to remove grit, oil and moisture. Black hot rolled steel (HRS) panels were washed with xylene and used without any mechanical cleaning to remove mill scale. A base coat as described in co-pending Indian Patent Application Number 202031031663 was applied on all types of panels to a DFT of 120-135 microns, air dried for a day and then top coated with the subject blue amino acrylic composition. The top coated panels were subjected to force drying at 70°C for 75 minutes after touch dry condition was achieved. The procedure followed was found to be equivalent to two days of air drying of the base coat and eight days of air drying after application of the top coat. The total DFT of all panels ranged from 200-220 microns and spray application was uniformly employed. a) Drying Time of top coat, maximum i) Tack free: 4 hours at 30°C ii) hard dry: 8 hours at 30°C iii) full cure: 8 days at 30°C. b) Adhesion: All types of coated panels showed excellent adhesion passing ASTM D3359, cross hatch with 5A, and 2 mm grid made using a single edge cutter with 5B. c) Hardness: Metal scratch hardness wherein a sharp steel needle loaded with a specified weight is scratched on the coated surface and the minimum weight required to expose the metal underneath the coating is recorded as scratch hardness. All coated panels exhibited scratch hardness of 5Kg weight minimum. d) Recoat ability: Some coated panels were exposed to normally prevailing weather conditions for 6 months and recoated with the acrylic top coat alone after cleaning the surface with a dry cloth. All such panels showed good adhesion passing ASTM D3359 with classification 5A. e) Chemical Resistance: Coated panels were tested by a chemical spot test in accordance with ASTM D1308 (modified version incorporated in ASTM D3730) for 4 hours, with i)20% aqueous solution of acetic acid, ii)50% aqueous solution of sulphuric acid, iii)30% aqueous solution of hydrochloric acid, and iv)50% aqueous solution of sodium hydroxide. Results obtained were satisfactory as all tests produced no effect on the coated area. f) Corrosion Resistance (salt fog): Coated panels were scribed and exposed to neutral salt fog as per ASTM B117 using 5% NaCI aqueous solution. Both types of panel showed no corrosion on the coated face as well as the stripped metal face and less than 1.0 mm. creep at scribe after 1500 hours of continuous exposure. No swelling or blistering of coating was observed. g) Black reinforced bars of 8-12 mm diameter coated by coating scheme referred to above, without any sand/grit/shot blasting, to a total DFT of 150 microns approximately were subjected to 1000 hours of electrochemical impedance spectroscopy study using simulated concrete pore solution dosed with 1% NaCI. The polarization resistance (Rp) retained a value of 1500 Kohms.cm ² at the conclusion of the test which is comparable to performance of fusion bonded epoxy coated on blasted reinforced bars subjected to the same test.

Qualitative tests were conducted to determine retention of properties on outdoor exposure. No loss of colour through fading, no chalking or cracking, were observed after 10 months of constant exposure of top coated HRS and CRS panels. Also, no effect of exposure up to 120°C for long periods, sustained and periodic, was seen on coated panels. Results obtained from all tests above illustrate the utility of the amino acrylic coating individually and as a part of coating scheme, preferably using a surface tolerant base coat referred to earlier for corrosion protection of steel exposed to industrial pollutants and coastal salinity, without resorting to elaborate surface preparation.

While the invention has been described in detail and with reference to the specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made without deviating or departing from the spirit and scope of the invention. Thus the disclosure contained herein includes within its ambit the obvious equivalents and substitutes as well.

Having described the invention in detail with particular reference to the illustrative examples given above, it will be more particularly defined by claims appended hereafter.